1. Field of the Invention
The present invention generally relates to a heat-treating apparatus and, more particularly, to an apparatus for heat-treating a multiple of objects such as a plate or the like, for example wafer or base materials of semiconductor or liquid crystal devices, with a batch scheme.
2. Description of the Related Art
Generally, in the manufacturing process of a semiconductor device or a liquid crystal device (LCD), various types of heat treatment processes are practiced.
For example, in the film deposition step or thermal diffusion step of the manufacturing process of a semiconductor device, a batch-scheme heat-treating apparatus, in which a heater is arranged around a reaction tube for housing a multiple of semiconductor wafers as objects to be treated, is used.
In such a batch-scheme heat-treating apparatus, since uniformity of temperature atmosphere must be kept in arrangement positions of the semiconductor wafers, electrical heaters are divided into a plurality of regions, and temperature detectors, e.g., thermocouples are arranged at a plurality of positions outside or inside the reaction tube. Detection signals from the thermocouples are sequentially supplied to a temperature measuring circuit to measure the temperatures of the positions where the respective thermocouples are arranged. An AC power of the order of 100 V and 100 A to be applied to the heater is controlled based on the measured temperatures, thereby controlling the temperature inside the reaction tube to a predetermined value.
In this case, in order to improve the uniformity of the temperature profile in the reaction tube, the interior of the reaction tube is divided into a plurality of zones, and thermocouples are provided in the respective zones. The power to be applied to the heater is controlled in units of zones by temperature measurement by the thermocouples. In this case, a single thermocouple is provided to each zone.
When a thermocouple, as described above, is exposed to a high-temperature atmosphere, e.g., 1,000.degree. C. or more for a long period of time, damage such as a disconnection or short circuiting is caused due to so-called thermal fatigue. For this reason, although a thermocouple is generally housed in a ceramic protection tube, a sufficient protection is not obtained for the thermocouple against a high-temperature atmosphere, and the damage described above often occurs.
In a heat-treating apparatus which performs temperature control by a plurality of thermocouples as described above, assume that damage such as a disconnection or short circuiting occurs in a single thermocouple. Then, an abnormal power is supplied to the heater, or power supply to the heater is shut off. As a result, an atmosphere of a predetermined temperature for the object to be treated cannot be maintained, and semiconductor wafers per lot, e.g., as many as 100 wafers, can become entirely defective.
More particularly, a batch-scheme heat-treating apparatus is usually designed to be able to treat 100 to 200 semiconductor wafers per lot at a time. If all the semiconductor wafers of one lot become defective, a serious economical loss is caused.
During the heat treatment described above, assume that it is detected by some means that a thermocouple is damaged. It is impossible to stop the operation immediately and replace the damaged thermocouple with a normal thermocouple. Consequently, even if the damaged thermocouple is replaced, the semiconductor wafers may be inappropriately treated.
The voltage of a temperature detection signal obtained by a thermocouple, as described above, is as very low as between 10.degree. and 20.mu.V/.degree. C. (about 10.5 mV at 1,000.degree. C.). Therefore, the temperature detection signal is usually amplified by a DC amplifier to about several hundreds of times and supplied to an analog/digital (A/D) converter, and the temperature is measured based on the output of the A/D converter.
The output signal from the A/D converter based on the temperature detection signal obtained by a thermocouple, as described above, includes many noise components. It is especially influenced by the noise of the power supply of the AC power to be supplied to the heater. In addition, the temperature detection signal from a thermocouple has a very low voltage, as described above. Therefore, the measurement precision, and hence the temperature control precision, are largely degraded by the noise components.
In an ordinary heat-treating apparatus, after a temperature detection signal is amplified, it is filtered by, e.g., an RC filter comprising a resistor and a capacitor, in order to remove its noise components. In this method, however, when a filter having a capacitor of a large capacitance enough to provide a sufficient noise removing effect is used, a practical response characteristic as the heat-treating apparatus cannot be obtained. On the other hand, with a filter that can provide a practical response characteristic, high-precision temperature measurement and control cannot be achieved, and the objects to be treated become defective, resulting in an economical loss.
As described above, the temperature detection signals from the plurality of thermocouples are sequentially switched by, e.g., a multiplexer and supplied to a measuring circuit, thereby performing respective temperature measurements.
However, during temperature measurement of a plurality of positions, when the detection signals are switched by, e.g., the multiplexer as described above, a measurement error can be easily caused by, e.g., a temperature change in an ON or OFF resistance of the multiplexer, a thermo electromotive force generated by the metal heterojunction of a connector, or a drift of the amplifier caused during amplification of a detection signal. Especially, in the temperature measurement by the thermocouple, the temperature detection signal has a very small rate of increase in voltage between about 10 and 20 .mu.V/.degree. C. Therefore, even a slight thermo electromotive force can result in a very large measurement error. As a result, high-precision temperature measurement and control cannot be performed only by means of switching by a multiplexer, and the objects to be treated inevitably become defective, resulting in an economical loss.
These apply not only to a heat treatment of a wafer in the manufacturing process of a semiconductor device, but also to a heat treatment of an LCD base member in the manufacturing process of an LCD.